Hard workouts pay off: Metabolic rate can keep pace with vigor of exercise


December 14, 1993|By Dr. Gabe Mirkin | Dr. Gabe Mirkin,Contributing Writer United Features Syndicate

When you lie in bed at night, your body burns between 40 and 120 calories per hour, depending on how large you are and how active your metabolism is. Those with slow metabolism burn fewer calories sleeping than those who have a more active metabolism. People with the slowest metabolisms are usually the fattest, while those who burn the most calories sleeping are usually the thinnest.

Over the past 50 years, scientists have debated whether exercise increases metabolism. Recently, several studies have shown that leisurely exercise does not speed up your metabolism but vigorous exercise does. Evidently, a crucial factor in increasing your metabolism is that you exercise intensely enough to raise your body temperature. Swimming does not increase your metabolism because the water conducts heat so rapidly from your body that your temperature does not rise. Exercising vigorously on land does increase your metabolism.

If you want to use exercise to help lose weight and keep it off, you should exercise at a faster pace once or twice a week. Spend a couple of months working up to 30 minutes of continuous exercise at least three or four times per week. Then try this workout: Start out slowly and gradually increase the pace until you feel uncomfortable or short of breath, slow down until you recover, then gradually increase the pace again. Do this interval-type training until your muscles start to feel tired. Do this workout once or twice a week, but never on consecutive days. On the other five or six days, exercise at a leisurely pace or take a day off.

Q: Is there anything I can do to keep my feet from swelling when I have to stand for a long time?

A: Your leg muscles function as a second heart to pump fluid from your legs to your heart. When your leg muscles relax, the veins near them fill up with blood. When your leg muscles contract, they compress the veins and squeeze blood up toward your heart.

When you stand still, your heart has to work very hard to pump blood against gravity from your feet to your heart. When your feet are above your heart, gravity works with you to help blood and fluid return to the heart. Eight hours of standing or sitting causes your feet to swell up to more than 110 percent of their normal size. This can make your shoes feel tight and your feet hurt.

The best way to prevent swelling is to elevate your feet. The next best way is to move your feet and toes frequently while you are sitting or standing. This will reduce swelling by more than 50 percent and will usually prevent the pain that it causes.

Q: Is it true that a lot of exercise can make you sterile?

A: Exercise can make you feel more sexual, but too much exercise can make you temporarily sterile. Sexuality starts in the brain. The brain produces a hormone (called GnRH) that causes the ovaries to produce the female hormone, estrogen and the testes to produce the male hormone, testosterone.

Heavy exercise can reduce brain levels of GnRH, and that, in turn, lowers blood levels of testosterone and estrogen. It is perfectly obvious when this happens to a woman. She will develop irregular periods or stop menstruating altogether, which causes her to stop releasing eggs into her uterus so she is temporarily unable to become pregnant.

It's not so obvious when a man becomes sterile. To see if a man is infertile, his semen must be checked under a microscope for decreased sperm numbers and activity. It can take several weeks for low sperm counts caused by excessive exercise to return to normal.

The vast majority of athletes in hard training can have children. Infertility occurs only when you train too much. You can tell when you are training too much because your muscles feel heavy and hurt, your joints ache and you feel tired all the time.

Don't count on the overtraining syndrome as a contraceptive. Women who have not been releasing eggs can suddenly release one and men with low sperm counts can have them return to normal without warning. While exercise offers many benefits, you can't depend on it for everything.

Dr. Mirkin is a practicing physician in Silver Spring specializing in sports medicine and nutrition.

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